= Exchange-positive CP convention for neutral kaon mixing
{title2=$|K_\pm\rangle=\frac{|K_{1,2}\rangle+\epsilon|K_{2,1}\rangle}{\sqrt{1+|\epsilon|^2}}$}
In the phase convention $CP|K^0\rangle=|\bar K^0\rangle$, the symmetric flavor combination is CP even and the antisymmetric one CP odd. For an equal-diagonal effective matrix with entries b,c off diagonal, choose correlated roots r,s with $r^2=b$, $s^2=c$ and eigenvalues $a\pm rs$. Its eigenvectors are $(r,\pm s)^T$. Writing $\epsilon=(r-s)/(r+s)$ gives the displayed CP-basis mixtures, up to an overall phase. Their deviation is small only if $|\epsilon|\ll1$; noninvariance alone does not guarantee that. This convention must not be mixed with one in which flavor exchange carries a minus sign.
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